1. Field of the Invention
The present invention relates generally to medical devices, and more particularly to patch electrodes for use with automatic implantable cardiac defibrillators.
2. Relevant Background
Cardiac arrhythmias can arise in the atria or ventricles as a consequence of an impairment of the heart's electrophysiologic properties such as excitability, conductivity, and automaticity (rhythmicity). Tachycardia is an arrhythmia characterized by rapid beating of the affected chamber which, in some instances, may lead to fibrillation. During fibrillation, sections of conductive cardiac tissue of the affected chamber undergo completely uncoordinated random contractions, quickly resulting in a complete loss of synchronous contraction of the overall mass of tissue and a consequent loss of the blood-pumping capability of that chamber.
Because of the lack of contribution of the atrial chambers to cardiac output, atrial fibrillation is hemodynamically tolerated and not generally regarded as life-threatening. However, in the case of ventricular fibrillation, cardiac output ceases instantaneously as a result of the rapid, chaotic electrical and mechanical activity of the excitable myocardial tissue and the consequent ineffectual quivering of the ventricles. Unless cardiac output is restored almost immediately after the onset of ventricular fibrillation, tissue begins to die for lack of oxygenated blood, and death will occur within minutes.
Ventricular fibrillation is frequently triggered by acceleration of a ventricular tachycardia. Hence, various methods and devices have been developed or proposed to treat and arrest the tachycardia before the onset of fibrillation. Conventional techniques for terminating tachycardia include pacing therapy and cardioversion. In the latter technique, the heart is shocked with one or more current or voltage pulses of generally considerably higher energy content than is delivered in pacing pulses. Unfortunately, the use of such therapy itself presents a considerable risk of precipitating fibrillation.
Defibrillation--that is, the method employed to terminate fibrillation--involves applying one or more high energy "countershocks" to the heart in an effort to overwhelm the chaotic contractions of individual tissue sections and to re-establish an organized spreading of action potential from cell to cell of the myocardium, thereby restoring the synchronized contraction of the mass of tissue. If these chaotic contractions continue in any tissue section, the defibrillation may be short-lived in that the uncontrolled tissue section remains a potential source for re-fibrillation. Successful defibrillation clearly requires the delivery of a shocking pulse containing a substantial amount of electrical energy to the heart of the afflicted person, at least adequate to terminate the fibrillation and the preclude an immediate re-emergence.
In the conventional approach of transthoracic external defibrillation, paddles are positioned on the patient's thorax and, typically, from about 100 to about 400 joules of electrical energy is delivered to the chest area in the region of the heart. It is apparent from the manner in which the shock is applied that only a portion of this energy is actually delivered to the heart and, thus, is available to arrest fibrillation. Where fibrillation occurs during open heart surgery, internal paddles may be applied to opposite surfaces of the ventricular myocardium and, in these instances, the energy required to be delivered is considerably less, on the order of 20 to 40 joules.
Over the past several years, implantable automatic defibrillators have been developed for use in detecting and treating ventricular fibrillation. In 1970, M. Mirowski et al. and J. C. Schuder et al. separately reported in the scientific literature their independent proposals for a "standby automatic defibrillator" and a "completely implanted defibrillator", respectively, including experimental results in dog tests. Since that time, a vast number of improvements in implantatable defibrillators, including electrode placement using extraperidcardial patches or transvenous catheter, has been reported in the scientific literature and patent publications.
The pulse energy requirements for internal defibrillation with known implantatable defibrillators and electrode systems range from about 5 joules to approximately 40 joules. Of course, the actual energy level required may differ from patient to patient, and further depends on such factors as the type of pulse waveform and the electrode configuration employed. While advances and improvements in electrical energy sources in general and pacemaker batteries in particular have been made over the past few years, it is clear, nonetheless, that repeated delivery of such amounts of energy from an implanted system will deplete conventional batteries in relatively short order. Accordingly, reduction of the energy level required for internal defibrillation remains a key area of inquiry and investigation.
It is a principal object of the present invention to provide improvements in electrode systems for internal defibrillation and in methods for making such electrode systems.
A related object is to provide an implantable electrode system, for use with implantable automatic defibrillators, which delivers the defibrillating waveform with considerably greater efficiency than has been achieved using prior art systems, and which therefore provides successful defibrillation at markedly reduced levels of electrical energy, compared to the levels heretofore necessary.
An early U.S. patent, in terms of the relative immaturity of developments in the field, U.S. Pat. No. 2,985,172, issued in 1961, described a tissue contact electrode for use in delivering a high voltage discharge directly to the heart. Each electrode consisted of a conductive ring connected to an insulated electrical lead, the ring holding conductive foil members and enclosed in a gauze sock, with a flexible backing member at one side of the gauze sheath. The overall electrode pad was described as sufficiently flexible to assume a dished shape tightly engaging the tissue of the heart.
In U.S. Pat. No. 4,030,509, Heilman et al. described an implantable electrode system for ventricular defibrillation, in which the electrodes are arranged in a generally base-apex configuration with a split conformal base electrode positioned above the base of the ventricles in the region of the atria, and a cuplike conformal apex electrode positioned at the apex of the heart.
In U.S. Pat. Nos. 4,270,549 and 4,291,707, Heilman et al. disclosed defibrillation electrodes of rectangular shape designed for insertion through the soft tissues outside the pleural cavity for contacting the heart. Each electrode consists of a metallic mesh either sandwiched between two layers of inert electrical insulation material or backed with a single layer of such material stitched to the mesh.
In U.S. Pat. No. 4,548,203, Tacker et al. disclosed an electrode system for use with implantable pulse generators employed for cardioversion or defibrillation. The system consists of two sets of opposed patch electrodes, one pair disposed laterally on the epicardium and the other pair disposed ventrally-dorsally, with each electrode orthogonal to the adjacent electrodes. The patent asserts that the presence of the latter pair of electrodes does not significantly alter the current distribution from the first pair, so long as the electrodes are relatively small with respect to the epicardial circumference and the two pairs are isolated from each other during current flow. The patent further ascribes the use of two pairs of electrodes implanted in spaced relationship as purportedly permitting the use of smaller electrodes, lower voltage and current, and lower total energy, with a more uniform current density and less hazard of damage to adjacent heart tissue, than had theretofore been achieved. Two current pulses are sequentially delivered to the separate pairs of electrodes to provide a temporal and spatial summation effect for the defibrillating current.
Nevertheless, these and other prior art electrode systems proposed for use with implantable defibrillators dissipate relatively large amounts of energy in delivering shocking pulses to the heart. Consequently, their performance tends to be marred by unacceptably high risk of damage to the myocardium, and by the need for relatively large sized defibrillation pulse generators (including batteries) to supply the high energy levels required over even somewhat short term use.
A reduction in the shock strength required for defibrillation would be advantageous in that it would allow for a decrease in the size of the automatic implantable defibrillator, an increase in battery life, and a reduction of the possibility of myocardial damage resulting from the shock.